lavf: the core rfps code needs 2 durations to estimate rfps

This fixes a regression where this count became 1 with
HPM-GC\ EXPORT\ FCP-1A-AVCI100-1080i25-001.mxf
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2884,7 +2884,7 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
             // ipmovie.c produces.
             if (tb_unreliable(st->codec) && st->info->duration_count > 15 && st->info->duration_gcd > FFMAX(1, st->time_base.den/(500LL*st->time_base.num)) && !st->r_frame_rate.num)
                 av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den, st->time_base.den, st->time_base.num * st->info->duration_gcd, INT_MAX);
-            if (st->info->duration_count && !st->r_frame_rate.num
+            if (st->info->duration_count>1 && !st->r_frame_rate.num
                 && tb_unreliable(st->codec)) {
                 int num = 0;
                 double best_error= 0.01;